The outer-most P–M strength curve in Fig. 15 (solid line)was obtained fromthe strain compatibilitymethod by using themeasured stress–strain curve of HSA800 reported in Fig. 6. The experimental P–M interaction strength is very accurately predicted with the strain compatibility method. To construct the strain–compatibility based P–M interaction
curve in Fig. 15, the following procedure was used. First, the measured stress–strain relation of HSA800 was approximated as bilinear (see Fig. 16). The strain profile was assumed to be linear and the stress distribution at ultimate state was calculated with assuming that available extreme fiber strain is 5% (or ε2 in Fig. 16). Sectional P–M strength at a particular plastic neutral axis location was then calculated by integrating the corresponding stress profile (see Fig. 17). The plastic neutral axis location was varied fromthe centroid of the section to the outward direction such that the whole P–M interaction range could be covered.